In Escherichia coli B/r, metabolism of L-arabinose involves the induced synthesis of three enzymes, L-arabinose isomerase, L- ribulokinase, and L-ribulose-5-phosphate 4-epimerase, coded by three contiguous structural genes, A, B, and D, of the L-arabinose operon. Genetic studies have shown that this operon is under positve control. The present project is primarily a structural study of these three enzymes, in order to fully characterize the three gene products, to compare the molar ratios of the products formed under balanced growth conditions, and to provide a molecular basis for studies on the effect of mutations.